I'm getting products (from a specific category) using a custom query_posts() function. I limited the showposts parameter to 25, so how do I know to activate pagination if there are still products (posts) to load?

The correct reset function for a custom query via a new WP_Query object is wp_reset_postdata(). Refer to the codex or to Rarst's flowchart you linked to. Fixed. Also, while I was at it, note that "fields of query" are properties.
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Johannes PilleJun 13 '12 at 12:02